Thermochimica Acta 676 (2019) 33–38
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Thermo-optical properties as complementary parameters for damage
assessment of mineral oils aged under controlled conditions used in power
transformers
T
A. Marín-Serranoa, J.A. Balderas-Lópezb, P.A. Calvaa, , A. Aranda-Péreza
⁎
a
Instituto Politécnico Nacional, Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas (UPIITA), Av. Instituto Politécnico Nacional 2580, Gustavo A.
Madero, Col. Barrio La Laguna Ticomán, 07340 CDMX, Mexico
b
Instituto Politécnico Nacional, Unidad Profesional Interdisciplinaria de Biotecnología (UPIBI), Av. Acueducto S/N, Col. Barrio la Laguna Ticomán, C. P. 07340, CDMX,
Mexico
A R TICL E INFO
A BSTR A CT
PACS:
84.70.+p
77.22.Jp
Mineral oils are used in electric power transformers for their excellent dielectric properties. The continuous
exposition to electrical discharges and thermal stresses may cause chemical adulteration in these substances
which eventually could be cause of electrical catastrophic fails in the transformers. It is then important to
evaluate periodically the quality of the oils to prevent major damages. The technique currently used for such
purposes is gas chromatography, however its high inaccuracy iswell known. Chemical transformation of aged
oils changes their thermal and optical properties and these could be used as alternative parameters for diagnostic
purposes of electric power transformers. In this paper two configurations of photopyroelectric techniques were
used for optical absorption coefficient (at 405 nm) and thermal diffusivity measurements of mineral oils aged up
to 2300 h under controlled conditions. It was found that, at least up to this period of aging time, thermal
diffusivity does not show significant differences, optical absorption coefficient at 405 nm, on the other hand,
showed significant differences starting at 1000 h and then this optical parameter can potentially be used for
diagnostic purposes.
Keywords:
Mineral oils
Gas chromatography
Photopyroelectric techniques
Power transformers
1. Introduction
Mineral oils are widely used as electrical insulators due to their high
dielectric strength. Hence, these liquids are essential constituents in the
isolation system of electric power transformers, which mainly consists
of paper immersed in the mineral oil. Under working conditions this
isolation system is subjected to high electrical efforts that cause chemical degradation of the paper and oil by hydrolysis and oxidation.
Most frequent fails of electric power transformer are the result of the
chemical transformation of these two constituents [1–3]. Chemical
decomposition of the oil generates gases like hydrogen, methane,
acetylene, ethylene and ethane, degradation of paper, on the other
hand, generates carbon monoxide, carbon dioxide and furanic derivatives. On a difference of the oil, which can be changed, paper cannot be
replaced whereby its damage is a major problem in power transformers
and this is the most critical fail of this devices. Failures in transformers
are the cause of considerable losses in the electrical industry [4], by
which preventive maintenance is very important. Usual tests involve
⁎
detection of the gases which are produced during the operation of
transformers and gas chromatography that is the standardized methodology in the electrical industry [5]. The central idea of using this
analytical methodology is to study the gas development due to gradual
chemical adulteration of the oils. More complete characterization involves other methodologies like neutralization number and polymerization degree, which revealed the chemical changes on the oil and
paper, respectively. These techniques are expensive and time consuming whereby the search of other experimental criteria for the diagnosis of the quality of the insulation systems are relevant. Among
other alternatives for the study of the degree of degradation of the oils
are thermal analysis [6] and infrared spectroscopy [7] which involves
qualitative analysis of new chemical species as results of the chemical
degradation. Chemical adulteration of the oil due to aging may also
changed thermal and optical properties at such extent that measurement of thermal conductivity or thermal diffusivity and optical absorption coefficients (at convenient wave-lengths) could be used as
complementary parameters for better diagnostics of the state of electric
Corresponding author.
E-mail addresses: primoalbertocch@hotmail.com, pcalva@ipn.mx (P.A. Calva).
https://doi.org/10.1016/j.tca.2019.03.025
Received 12 December 2018; Received in revised form 12 February 2019; Accepted 19 March 2019
Available online 20 March 2019
0040-6031/ © 2019 Elsevier B.V. All rights reserved.
Thermochimica Acta 676 (2019) 33–38
A. Marín-Serrano, et al.
Fig. 1. PPE experimental setup for thermal diffusivity measurements of liquids.
Fig. 2. PPE experimental setup for optical absorption coefficient measurements
of liquids.
power transformers. Photothermal (PT) techniques have already been
used for diagnostic purposes for oils [8–12], in this paper two configurations of photopyroelectric (PPE) technique [11,12] are used for the
direct measurement of thermal diffusivity and optical absorption coefficient (at 405 nm) of aged oils up to 2300 h under controlled conditions. Some preliminary diagnostics about the quality of the oils are
provided based on these thermo-physical properties supported at some
extent on other more conventional studies as infrared spectroscopy.
where D is a constant, independent of the sample´s thickness and βs is
the optical absorption coefficient of the sample at the wave-length used
for the analysis (a diode laser pointer at 405 nm in this case). The optical absorption coefficient of the sample is obtained directly as the
slope of the linear fit of the PPE amplitude (in semi-log scale) as a
function of the sample thickness. The constant behavior of the PPE
phase is used in this case as experimental criterion for validate this
measurement. [12]
Systems for controlled degradation of the mineral oil
To thermally deteriorate the oil, two identical aging tanks of 25
liters capacity were used, in which copper-paper coils were placed inside, representing a scale model of a power transformer of 75 MVA,
keeping the copper/paper/oil relations. Fig. 3 is a photograph from de
outside of the experimental system for the aging of the dielectric mineral oils at controlled thermal stress and Fig. 4 is a photograph of one
coil inside one of the tanks.
Fig. 5 shows the schematic diagram of the experimental arrangement. The controlled deterioration of the paper-oil system was carried
out for a period of 2300 h at an average coil temperature, known as the
hottest point or 140 °C hotspot, with oil circulation rate of 1 L per
minute and with samplings at 300-hour intervals.
2. Experimental setups
2.1. Photopyroelectric setups for thermo-optical characterization
The PT techniques described in this paper for the assessment of the
thermal and optical properties of mineral oils have been published
elsewhere for which just some relevant aspects of them are described
here. Fig. 1 is a cross section of the PPE set up for thermal diffusivity
measurements [11], it consists of a diode laser (Thorlabs, model
L785P090, 90 mW CW power, λ = 785 nm) and a PVDF film as a
pyroelectric sensor (0.0052 cm thickness). Laser radiation, modulated
at 1 Hz, is absorbed on a thin silicon slab to generate thermal waves
(temperature fluctuations inside the silicon slab) which travels through
the liquid sample (thickness L), to reach the pyroelectric sensor. Under
the limiting condition of thermally thick regime of the liquid sample,
| s L|
1 (where
s
= (1 + i )
f
s
3. Samples preparation
, and αs is the thermal diffusivity of the
sample), the pyroelectric signal, as a function of the sample´s thickness,
is given by
V (L) = Ce
sL
The mineral oil used for this work was the NYNAS NITRO 11GBXUS, which is not-inhibited naphthenic oil. To extract the moisture
(1)
where C is a complex expression, independent of L. The thermal diffusivity of the sample can be obtained from the amplitude and phase of
Eq. (1) as the slope, M, of the linear fit of the PPE amplitude (in semilog scale) or phase, as a function of the sample thickness, by means of
f
the equation = 2 [11].
M
Fig. 2 shows a cross section of the PPE setup for optical absorption
coefficient measurements [12]. It is essentially the same as the one for
thermal diffusivity measurements but, on a difference of it, the silicon
slab is changed by an optical window as to allow light goes through the
liquid sample. In the thermally thick regime for the liquid sample the
pyroelectric signal is given in this case as
V (L) = De
sL
Fig. 3. Experimental arrangement system for aging of the dielectric mineral oil.
(2)
34
Thermochimica Acta 676 (2019) 33–38
A. Marín-Serrano, et al.
Table 1
Dissolved gasses inside oil samples obtained from different power Transformers
under real working conditions. Gas concentrations are given in ppm.
Fig. 4. Coil inside the tank shown in Fig. 3.
Gases
Sample R1
3 years aging time
Sample R2
4 years
aging time
Sample
R3
5 years aging time
H2
O2
N2
CH4
CO
CO2
C2H4
C2H6
C2H2
0.000
27722.72
70183.87
1.392
8.667
341.630
1.402
0.804
0.000
0.000
33668.38
69446.05
0.763
4.747
147.649
0.058
0.000
0.000
0.000
20235.85
51321.02
0.000
4.185
59.499
0.000
0.000
0.000
degradation of the oil and possibly by the affectation of the cellulose.
The dissolved gases that appear can be caused by thermal stresses due
to overloads, electrical stresses due to corona discharges and / or the
presence of electric arcs, moisture reactions with the air gap, among
other factors. The state of the transformers depends not only on the
time of service but also on how they are operated. Consequently in
addition to the oil samples obtained with the procedure described
above, three oil samples were obtained from different power transformers under real working conditions with operation times of 3, 4 and 5
years, and according to their gas chromatography, they reported the
dissolved gases shown in Table 1. It is pertinent to anticipate that
samples prepared with those obtained from real conditions are not
comparable.
Fig. 5. Experimental diagram of deterioration system of the paper-oil system.
absorbed by the paper that covers the coils, these were entered into an
autoclave system. The coils were subjected to a drying process at 100 °C
(212 °F) under vacuum conditions for 120 h, after which it was possible
to obtain a relative humidity in the paper of 0.75%. The coils were
impregnated with low moisture mineral dielectric oil (15 ppm) and the
thermocouples were connected to their respective terminals.
Afterwards, the aging containers were sealed and filled with virgin
mineral oil up to 90% of their capacity, keeping the terminals of the
current bushing (passage-walls) immersed in the fluid. After filling, the
air headspace was replaced with dry nitrogen until reaching 4 PSIG to
prevent oil absorbing moisture from the air. After a leak-proof and
stabilization time of 36 h under nitrogen pressure, the coil current was
increased using an autotransformer for each one. It was necessary to
increase the current intensity of the secondary at intervals of 10 amperes per minute until reaching the point at which the current stabilized
and the temperature of both coils was maintained at 140 ± 2 °C. Once
reached the temperature of 140 °C, the oil pumping device was put into
operation to homogenize the temperature of the oil in its entirety. At
the end of the 2300 h of aging, the coloration of the oil changed, from
being virtually transparent to presenting a bright yellow color, which
makes the oxidation evident as a consequence of the accelerated aging
to which it was subjected. Fig. 6 shows the change in coloration of the
oil.
It is well known that in real operating conditions of electrical power
transformers, it usually takes at least two years of continuous operation
to be able to observe, through conventional techniques, significant
deteriorations in the insulation systems, generated mainly by the
4. Results and analysis
4.1. UV vis characterization
Oil samples aged under controlled conditions and three oil samples
aged under real power transformer conditions were used for this study
(Table 2, column 1). Fig. 7 shows the UV–vis spectra, for the three last
samples (R1, R2 and R3).
A strong absorption band around 400 nm is evident for all samples,
showing the feasibility of using optical properties in the near UV as
quantitative magnitudes for aging estimation of these kind of substances. It is important to note that even though this procedure could be
carried out using the absorbance at a selected wave-length (400 nm for
instance), this is not reliable since there is no certainty that this magnitude complies with Beer-Lambert law for light absorption. As it has
been reported in the literature [12], the photopyroelectric technique
reported to carry out the measurement of the optical absorption
Table 2
Thermal diffusivity, αs, and optical absorption coefficient, βs (at 405 nm),
measured by the PPE techniques described in this work, for the mineral oils
samples aged under controlled (samples S1 to S7) and real working conditions
(samples R1, R2 and R3).
Fig. 6. Coloring of the oil at: A) 0 h; B) 2300 h of deterioration.
35
Oil
sample
βs (cm−1)
(405 nm)
αsx10−8
(m2/s)
S0 (0 h)
S1 (400 h)
S2 (700 h)
S3(1000 h)
S4 (1300 h)
S5 (1600 h)
S6 (1900 h)
S7 (2300 h)
R1 (3 years)
R2 (4 years)
R3 (5 years)
0.16
0.12
0.17
0.11
0.23
0.25
0.34
0.41
0.66
2.22
3.92
6.69
6.63
6.56
6.58
6.68
6.63
6.53
6.62
6.65
7.20
7.43
±
±
±
±
±
±
±
±
±
±
±
0.02
0.01
0.02
0.03
0.03
0.02
0.03
0.02
0.01
0.02
0.02
±
±
±
±
±
±
±
±
±
±
±
0.06
0.06
0.05
0.06
0.05
0.05
0.06
0.06
0.04
0.07
0.06
Thermochimica Acta 676 (2019) 33–38
A. Marín-Serrano, et al.
Fig. 9. Optical absorption coefficient, as a function of aging time, for the oil
samples aged under controlled conditions in a range of 0–2300 h.
Fig. 7. UV–vis spectra of the oils aged under real working conditions.
It can be observed that it experiments appreciable changes starting
from 1000 h of aging. These changes can be attached to the chemical
degradation of the oils which involves conversion of chemical species
inside the sample. It can be concluded that, at least for oils under this
controlled conditions, appreciable chemical conversion is experimented
up to 1000 h of aging. Qualitative determination of these chemical
modifications to the oil molecules due to artificial aging was made
using infrared spectroscopy. Fig. 10 shows the spectra obtained for the
samples aging between 400 and 2300 h and the corresponding one for
the sample without aging. The characteristics peaks for the functional
groups in this figure were identified based on the nature of the oil
molecule (oil for transformers of the brand NYNAS and naphthenic
base). According to literature [13] the oil without aging is mainly
composed of biphenol, naphthene and fluorene and this information
was fed to the special software for the analysis (KnowItAll software)
and using IR data tables. [14,15]
The triplet between 2800 cm−1 and 3000 cm−1, corresponds to the
CeH functional group as part of the fluorine. The doublet between
2310 cm−1 and 2380 cm−1, corresponds to the double bonds in the CO2
molecule and the peak located at 1730 cm−1 is related to the C]O
bond. Fig. 10 shows that CO2 is a product of degradation of the oils
since the corresponding peaks are evident starting at 1300 h of aging,
time at which the corresponding peak for the C]O bond starts to decrease.
The peak at 1315 cm−1 is related to the CeN bond; the presence of
this nitrogen bond in the aged oils is attributed to chemical
Fig. 8. Photopyroelectric phase, a, and amplitude (in semi-log scale), b, as a
function of the sample thickness for an oil sample aged at 1600 h under controlled conditions. The continuous line in Fig. 8b corresponds to the linear fit
for obtaining the coefficient of optical absorption of the sample at 405 nm. The
measured value of this optical parameter is shown on the same plot.
coefficient does not suffer from this deficiency since each measurement
of this parameter is validated by the constant phase condition in the
experiment. Taking into account this fact the optical absorption coefficients for all samples were obtained at 405 nm, this particular wavelength was chosen for this optical measurement since diode laser at this
wave-length is commercially available. As an example Fig. 8 shows
typical photopyroelectric signals, as function of the sample thickness,
for a sample at 1600 h of controlled aging. The phase constant in Fig. 8a
guarantee that the optical absorption coefficient, obtained as the slope
of the linear fit in Fig. 8b, fulfill the Beer-Lambert model for light absorption. Table 2, column 2 summaries the corresponding results for all
samples.
Fig. 9 shows the behavior of this optical parameter as function of the
time of aging.
Fig. 10. Infrared spectroscopy of aged oil from 400 to 2300 h.
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Thermochimica Acta 676 (2019) 33–38
A. Marín-Serrano, et al.
Table 3
Effect of thermal aging on breakdown voltages.
Aging time [h]
Breakdown voltage [kV]
0
2400
33.94
20.88
absorption coefficient (Fig. 9), there is not an appreciable change on
this thermal property over this period of aging time (1300 h) under
these controlled conditions. However appreciable changes on this
thermal property are evident for the samples obtained from transformer
under real working conditions which where aging for more than 3
years, as it is evident by looking at the two last rows on Table 2. It is
also evident that these changes are even larger for the optical absorption coefficient at 405 nm. This could be due to competitive behavior of
the multiple new compounds which are generated during the aging
process of the oil. Some of these compounds could promote and some
other disturbs the heat transfer inside the oils. At relatively small periods of time (less than 1300 h) these differences in thermal transfer
could be matched resulting in an overall null increment on thermal
diffusivity; however this matching could be broken at larger periods of
time, as it is shown in Table 2. The increase in the optical absorption
coefficient at 405 nm, on the other hand, could be due to the increase in
special chemical radical groups that promoted the UV radiation absorption and this fact increase monotonically with the aging time. To
figure out on these assumptions larger periods of aging time under
controlled conditions are required and it is part of another project.
Finally, one of the dielectric properties with greater dependence on
the physical-chemical characteristics of the oil is the dielectric breakdown voltage. This is indicative of the ability of the insulating liquid to
withstand electrical stress. The presence of any of the mechanisms of
deterioration of the oil, such as oxidation, is immediately reflected in its
value of dielectric strength. For this work the dielectric breakdown
voltages of the accelerating aging samples were determined with 0 h
and 2400 h of deterioration, respectively. These values were obtained
following the guidelines of the applicable regulations [16] and are
shown in Table 3.
Fig. 11. Photopyroelectric amplitude, as a function of the sample thickness for
an oil sample aged at 0 h. The continuous line corresponds to the linear fit for
obtaining the thermal diffusivity of the sample. The measured value of this
thermal property is shown.
Fig. 12. Thermal diffusivity, as a function of aging time, for the oil samples
aged under controlled conditions in a range of 0–2300 h.
combination with atmospheric nitrogen promoted by the electrical
discharges (in the experimental container the air was substituted by dry
nitrogen at a pressure of 4 PSI). It is interesting to note in Fig. 10 that
the corresponding peak gradually increase, reaching its maximum
around 1300 h of aging, and then decrease and disappear at 2300 h.
Chemical degradation of the oil is evident from these IR spectra by
looking at the peaks corresponding to CeN and C]O bonds. These facts
correlate qualitatively well with the optical absorption measurements
(Fig. 9) since at 1300 h the optical absorption coefficient at 405 nm also
starts to increase.
It is interesting to note that some peaks in Fig. 10 remain constant
during the aging process; this is an indication that, at least under these
controlled conditions, some molecular species inside the mineral oil
remains unchanged.
The type of gases dissolved in the volume of the oil and the associated chemical changes depend on the thermal and electrical stresses
to which it is subjected. These global changes eventually will play on
detectable changes on its thermal properties. To figure out about this
effect pyroelectric technique was used for the measurement of this
thermal property for all samples. Fig. 11 shows the photopyroelectric
amplitude, as a function of the sample thickness, for the oil sample aged
at 0 h for the goal of illustrate the analytical procedure for obtaining the
thermal diffusivity of the sample for all samples. Table 2, column 3,
summarized the thermal diffusivity values for all of the samples studied
in this work.
Thermal diffusivity as a function of the time of aging is shown in
Fig. 12. It is evident from this figure that, at a difference of the optical
5. Conclusions
For the first time photopyroelectric techniques were used for obtaining optical absorption coefficient and thermal diffusivity measurements of aged mineral oil from power transformers. It was found that
optical absorption coefficient, at 405 nm, of the aged oil can be used as
a complementary parameter to diagnose the state of the insulations.
The optical photopyroelectric analysis described in this work could be
easily carried out at other convenient wave-lengths (in the UV-region)
as to provide of more optical information and relate it with the different
chemical species which are formed as result of the degradation process.
Acknowledgements
This work was supported by the Instituto Politécnico Nacional
(Grant 2016812) and the Instituto Nacional de Electricidad y Energías
Limpias, of México.
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